A wide variety of off-line power supplies for providing power to LEDs are known. Many of these power supplies (i.e., drivers) are effectively incompatible with the existing lighting system infrastructure, such as the lighting systems typically utilized for incandescent or fluorescent lighting, such as infrastructure generally utilizing phase-modulating “dimmer” switches to alter the brightness or intensity of light output from incandescent bulbs. Accordingly, replacement of incandescent lamps by LEDs is facing a challenge: either do a complete rewiring of the lighting infrastructure, which is expensive and unlikely to occur, or develop new LED drivers compatible with commercially available and already installed dimmer switches. In addition, as many incandescent or other lamps will likely remain in any given lighting environment, it would be highly desirable to enable LEDs and incandescent lamps to be able to operate in parallel and under common control.
Incandescent lamps and LEDs can be connected to a common lamp power bus, with the light output intensity controlled using a composite waveform, having two power components. This is complicated, requires excessively many components to implement, and is not particularly oriented to AC (alternating current) utility lighting.
An off-line LED driver with a power factor correction capability has been described. When coupled with a dimmer, however, its LED regulation is poor and it does not completely support stable operation of the dimmer in the full range of output loads, specifically when both incandescent and LED lamps are being used in parallel.
FIG. 1 is a circuit diagram of a prior art current regulator 50 connected to a dimmer switch 75 which provides phase modulation. FIG. 2 is a circuit diagram of such a prior art (forward) dimmer switch 75. The time constant of resistor 76 (R1) and capacitor 77 (C1) control the firing angle “a” (illustrated in FIG. 3) of the triac 80. The diac 85 is used to maximize symmetry between the firing angle for the positive and negative half cycles of the input AC line voltage 35. Capacitor 45 (C2) and inductor 40 (L1) form a low pass filter to help reduce noise generated by the dimmer switch 75. A triac 80 is a switching device effectively equivalent to reverse parallel Silicon Controlled Rectifiers (SCRs), sharing a common gate. The single SCR is a gate-controlled semiconductor that behaves like a diode when turned on. A signal at the gate 70 is used to turn the triac 80 on, and the load current is used to hold or keep the triac 80 on. Thus, the gate signal cannot turn an SCR off and it will remain on until the load current goes to zero. A triac 80 behaves like an SCR but conducts in both directions. Triacs are well known to have different turn-on thresholds for positive and negative conduction. This difference is usually minimized by using a diac 85 coupled to the gate 70 of the triac 80 to control the turn-on voltage of the triac 80.
Triacs 80 also have minimum latching and holding currents. The latching current is the minimum current to turn on the triac 80 when given a sufficient gate pulse. The holding current is the minimum current to hold the triac 80 in an on state once conducting. When the current drops below this holding current, the triac 80 will turn off. The latching current is typically higher than the holding current. For dimmer switches that use triacs, capable of switching 3 to 8 A for example, the holding and latching currents are on the order of 10 mA to about 70 mA, also, for example and without limitation.
The firing angle (α) of the triac 80 controls the delay from the zero crossing of the AC line, and is theoretically limited between 0° and 180°, with 0° equating to full power and 180° to no power delivered to the load, with an exemplary phase-modulated output voltage illustrated in FIG. 3 (as a “chopped” sinusoid). A typical dimmer switch, for example, may have minimum and maximum α values of about 25° and 155°, respectively, allowing about 98% to 2% of power to flow to the load compared to operation directly from the AC mains (AC line voltage 35). Similarly, a reverse-phase modulated dimmer will provide an output voltage across a resistive load as illustrated in FIG. 4, which provides energy to the load at the beginning of each cycle, such as from 0° to 90°, for example, with no energy delivered in the latter part of each cycle (illustrated as interval β).
Referring to FIG. 2, the firing angle α is determined by the RC time constant of capacitor 77 (C1), resistor 76 (R1), and the impedance of the load, such as an incandescent bulb or an LED driver circuit (ZLOAD 81). In typical dimming applications, ZLOAD will be orders of magnitude lower than R1 and resistive, thus will not affect the firing angle appreciably. When the load is comparable to R1 or is not resistive, however, the firing angle and behavior of the dimmer switch can change dramatically.
Typical prior art, off-line AC/DC converters that drive LEDs using phase modulation from a dimmer switch, have several problems associated with providing a quality drive to LEDs, such as (1) phase modulation from a dimmer switch can produce a low frequency (about 120 Hz) in the optical output, referred to as “flicker,” which can be detected by a human eye or otherwise create a reaction in people to the oscillating light; (2) filtering the input voltage may require quite a substantial value of the input capacitor, compromising both the size of the converter and its useful life; (3) when the triac 80 is turned on, a large inrush current may be created, due to a low impedance of the input filter, which may damage elements of both the dimmer switch 75 and any LED driver; and (4) power management controllers are typically not designed to operate in an environment having phase modulation of input voltage and could malfunction.